The present invention relates to a cell output control apparatus and method in communication systems using asynchronous transfer mode, and particularly to a cell output control circuit apparatus and method suitable for use in communication systems which provide available bit rate services.
The U.S. Pat. No. 5,313,454 has disclosed a conversion system having an available bit rate (herein-after, abbreviated ABR) traffic class incorporated in order to make feed-back control for preventing the congestion in a communications network in which cells of asynchronous transfer mode (hereinafter, abbreviated ATM) are processed. Here, the ABR traffic class is for transmitting and receiving ATM cells from a source (transmitting terminal, or a transmitting node) at an allowed cell rate (hereinafter, abbreviated ACR) in an ATM communications network. This system sets up such an ACR that when a resource management cell (hereinafter, abbreviated RM cell) including the congestion information of the network is inserted in the cell flow that is transmitted and received within the network, and transferred back to the source from the destination (receiving terminal, or receiving node), the source changes the ACR in accordance with the information of the received RM cell so that the congestion within the network can be decreased as much as possible and that the cells can be transmitted at a large transfer rate.
When the cells are transmitted from the source at the ACR, the ATM-communication system decides whether a cell is allowed to transmit at each ATM cell time slot or not in order to transmit at the ACR. In general, after a cell is transmitted on a certain cell time slot, the next cell time slot to be produced is calculated, and the cell is timely transmitted on that slot. More specifically, when the line cell rate (hereinafter, abbreviated LCR) and the ACR are 150 Mb/s and 50 Mb/s, respectively, a cell can be transmitted at each three time slots. Therefore, after a cell is transmitted on a certain time slot, the next time slot is produced three cell time slots later. In other words, when a cell is transmitted on a certain cell time slot, the next cell time slot is produced LCR/ACR cell time slots later.